1. FIELD OF THE INVENTION
The present invention relates to an elastic surface wave filter suitably adapted for use in a color television receiver.
2. DESCRIPTION OF THE PRIOR ART
In common color television receivers in the prior art, a high frequency television signal received by an antenna is supplied to a tuner and converted to a video intermediate frequency signal. The video intermediate frequency signal is amplified by a video intermediate frequency amplifier having a predetermined amplitude-frequency characteristic and then video-detected to be a color composite video signal. The color composite video signal of an NTSC system used, for example, in Japan or U.S.A. comprises a sound signal component with a sound carrier of 4.5 MHz frequency-modulated by a sound signal, a chromaticity signal component with a sub-carrier of 3.58 MHz modulated according to a quadrature phase modulation by red and blue color difference signals, and a luminance signal ranging in frequency from 0 to 4 MHz. The color composite video signal is fed to a sound channel including a narrow band pass filter for passing therethrough the sound signal component, an FM demodulator and the line, and also to a sound trap for attenuating the sound signal component. The color composite video signal with its sound signal component attenuated by the sound trap is supplied to a chromaticity channel including a band pass amplifier for passing therethrough the chromaticity signal component, a color difference signal demodulator and the like, and also to a luminance channel including a chromaticity signal trap and a delay line for compensating the difference of a delay time between the the chromaticity channel and luminance channel. Red, blue and green color difference signals fed out from the chromaticity channel and a luminance signal from the luminance channel are supplied to a matrix circuit and converted to red, blue and green primary colors each making primary colored fluorescent materials on the color picture tube luminescent.
The band pass amplifier in the chromaticity channel serves to pass therethrough the chromaticity signal component and prevent the passage of low frequency components in the luminance signal component, and thus has the pass band ranging from 3.08 to 4.08 MHz. The delay time of the luminance signal passing through the luminance channel is generally shorter by 0.4 to 1.0 .mu.s than that of the signals near to the color sub-carrier passing through the chromaticity channel because the luminance channel has a wider band than the chromaticity channel. For this reason, the luminance channel is provided with a delay line for compensating the difference between these delay times to remove the difference of the delay time between the luminance signal and the signal near to the color sub-carrier.
Such a color television receiver, however, has the drawback that the delay time of the chromaticity signal passing through the chromaticity channel depends upon its frequency with colors not fully reproducible on the screen of the receiver. In other words, the chromaticity signal component which has passed the sound trap is attenuated more strongly as its frequency has the component nearer to the frequency of the sound trap, i.e., as it has the higher frequency component, so that the band pass amplifier is so provided that it is free therefrom and produces an output having a flat amplitude characteristic. If the chromaticity signal passes through a transmission line having a non-uniform amplitude characteristic, this leads to a non-uniform group delay characteristic and the dependency of the delay time of the color signal component upon its frequency with the above-mentioned drawback. This fact is disclosed in "Colour Television", pages 113 and 114, written by P. S. Carnt, G. B. Townsend and published in 1968 by Iliffe Books Ltd. (London). In the associated pages of this book there is described the fact that the delay time of the chromaticity signal component depends upon its frequency, i.e., that is transmitted signal is provided with such a group delay characteristic as to compensate the non-uniformity of the group delay characteristic. It, however, has been found that the chromaticity signal has no uniform group delay characteristic even if the transmitted signal has its group delay characteristic changed.